Depression on River Plate (7)
I believe the answer is:
denture
'plate' is the definition.
(thesaurus)
'depression on river' is the wordplay.
'depression' becomes 'dent' ('dent' can be a synonym of 'depression'**).
'on' is a charade indicator (letters next to each other).
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'dent'+'ure'='DENTURE'
(Other definitions for denture that I've seen before include "False teeth plate" , "Set of false teeth" , "Plate with artificial teeth" , "Set of artificial teeth" .)